II.] 



THE SPINAL SKELETON. 



than in many Vertebrates, e.g. than in Bats and Birds. Their 

 occasionally bifid condition in man may be repeated in much 

 lower Vertebrates (e.g. Axolotl), and sometimes (as in Poly- 

 pterus] there may be two neural spines to one vertebra, one in 



FIG. 51. Dorsal view of Sixth, Seventh, 

 and Eighth Post Sacral Vertebrae of 

 the Axolotl, showing the laterally bi- 

 furcating neural spines, each concave 

 at its extremity. 



(From the College of Surgeons ' 

 Museum.) 



FIG. 52. Six TRUNK-VERTEBRA OF 



POLYPTERUS. 



The third and fourth vertebrae have 

 each two transverse processes and 

 four ribs on the side shown ; the 

 third vertebra has also two neural 

 spines, s, neural spine ; p, lower 

 ribs. The series of upper ribs is 

 not distinguished by any letter. 

 (From the College of Surgeons ' 

 Museum, ) 



the front (i.e. pre-axial) of the other. Indeed, in the Conger we 

 have two such projections from each side of the neural arch. 



In certain flat fishes they may be detached from the arches 

 and intercalated between them. They may expand and 

 simulate dermal scutes, as in the Tortoises. They may project 

 through the skin of the back, as in the Potto, or be produced 

 into long, free filamentary processes, as in the Lizards called 

 Basilisks. 



The intervertebral foramina of man are normal, but in 



FIG 53. Seven Trunk- Vertebrae of the Potto, showing the nervous perforations 

 in the neural laminae. 



some animals, even so nearly allied to man as the Potto, 

 also in the Horse, Ox, Monotremes, and others, the nerves 



